Objectives: (1) To investigate whether the DNA repair system in mouse germ cells can be utilized as a predictive test system for detection of environmental mutagens; (2) to study biochemical and toxic effects of environmental agents on mouse spermatogenesis using both velocity sedimentation cell separation and serial mating techniques; (3) to define enzyme development during spermatogenesis and attempt to develop useful "marker" enzymes to indicate normal cellular differentiation and toxic effects of environmental agents on mouse spermatogenesis: (4) to characterize the interrelationship between hormone action and repression of testicular enzyme synthesis; and (5) to investigate the nature of the blood-testis barrier and the penetration of environmental chemicals to rat spermatogenic cells.